Multispecies fish tracking across newly created shallow and deep habitats in a forward-restored lake

Abstract

Background: Freshwater fish communities typically thrive in heterogenous ecosystems that offer various abiotic conditions. However, human impact increasingly leads to loss of this natural heterogeneity and its associated rich fish communities. To reverse this trend, we need guidelines on how to effectively restore or recreate habitats for multiple fish species. Lake Markermeer in the Netherlands is a human-created 70,000-ha lake with a uniform 4 m-water depth, steep shorelines, high wind-induced turbidity, and a declining fish community. In 2016, a forward-looking restoration project newly created a 1000-ha five-island archipelago in this degrading lake, which offered new sheltered shallow waters and deep sand excavations to the fish community.

Methods: In 2020, we assessed how omnivorous and piscivorous fish species used these new habitats by tracking 78 adult fish of five key species across local and lake-scales. We monitored spring arrival of adult fish and assessed local macro-invertebrate and young-of-the-year fish densities.

Results: Adult omnivorous Cyprinidae and piscivorous Percidae arrived at the archipelago in early spring, corresponding with expected spawning movements. During the productive summer season, 12 species of young-of-the-year fish appeared along the sheltered shorelines, with particularly high densities of common roach (Rutilus rutilus) and European perch (Perca fluviatilis). This suggests the sheltered, shallow, vegetated waters formed new suitable spawning and recruitment habitat for the fish community. Despite highest food densities for adult fish in the shallowest habitats (< 2-m), adult fish preferred minimally 2-m deep water. After spawning most Cyprinidae left the archipelago and moved long distances through the lake system, while most Percidae remained resident. This may be related to (1) high densities of young-of-the-year fish as food for piscivores, (2) medium food densities for omnivores compared to elsewhere in the lake-system, or (3) the attractiveness of 30-m deep sand excavations that were newly created and frequently used by one-third of all tracked fish.

Conclusions: New littoral zones and a deep sand excavation constructed in a uniform shallow lake that lacked these habitat types attracted omnivorous and piscivorous fish species within four years. Both feeding guilds used the littoral zones for reproduction and nursery, and notably piscivorous fish became residents year-round.

Keywords: Acoustic telemetry; Deep sand excavation; Fish nursery; Forward-looking restoration; Macro-invertebrates; Marker Wadden; Shallow lake; Shelter.

Fig. 1

a The large-scale study area in the centre of the Netherlands (see inset) consisting of several connected waterbodies, including the focal lake Markermeer. Fish were monitored at large spatial scales through a network of acoustic receivers maintained by collaborators (n = 33, black triangles) throughout the lake system. The Marker Wadden archipelago is constructed in lake Markermeer, west of the ‘Houtribdijk’ dam that separates lake Markermeer and lake IJsselmeer. The Houtribdijk forms an incomplete boundary for fish with sluices at either end. b The Marker Wadden archipelago with sheltered gradual land–water transition shorelines leading into shallow littoral habitat, and the 30-m deep sand excavation pit created during the building of the archipelago on the southwest side. Fish were monitored at local spatial scales by a network of 13 acoustic receivers especially deployed for this study (black triangles). This created four sections varying in shelter and water depths; c Illustrative ground pictures of the four types of habitat indicated in panel b and as described in the methods

Fig. 2

a Young-of-the-year fish CPUE per 100 m shoreline sampled within the four habitat categories within the archipelago waters and along the exposed open lake shores. All sites were sampled from May until August, error bars denote standard errors across the sampled locations. Proportion of CPUE per focal fish species is indicated by colours. Letters denote significance of differences among habitat types based on generalized linear models (statistical details in Additional file 1: Table S2a). b Biomass of benthic and pelagic macro-invertebrates per surface area (0.20 m²), for the four categories of shorelines on a log10-scale. The proportional contributions of five macro-invertebrate families to the biomass are overlaid on the bars, which together represent 98.9% of all organisms caught. Letters denote significance of differences in total biomass among habitat types based on generalized linear models (statistical details in Additional file 1: Table S2b)

Fig. 3

Adult fish habitat use for the five study species at local spatial scales of the Marker Wadden archipelago, depicted as proportion of time spend at each location type (see Fig. 1b) over the year that the fishes were monitored. Note that the sections differed in total surface areas with the harbour being the smallest areas (~ 8 ha) followed by the marshes (~ 50 ha) and the centre as largest area (~ 80 ha); and note that the numbers of individuals on which the data are based declined over time in the season (see Fig. 4)

Fig. 4

Kaplan-Meijer survival curve displaying the number of days between the first and last detections of three Cyprinidae species and two Percidae species in the area (n = 78). The vertical axis displays the portion of the fish species still considered resident in the shallow sections of the archipelago (i.e. excluding visits to the sand excavation only). Percidae remained significantly longer around the archipelago than the Cyprinidae (Survival Analysis at the family level: P = 0.015, statistical details per species level in Additional file 1: Fig. S1)

Fig. 5

Lake-scale movements of adult fish of the five studied species in the large-scale study area with the network of receivers indicated with black triangles. Coloured circles indicate receivers that were visited by fish. The size of the circles is scaled to the number of detected individual fish. The widths of the connecting lines depict the number of unique movements between receivers for all fish of a species combined